Speed responsive apparatus



April 6 948-` L. HAMMoND Er Al. A 2,439,295

SPEED RESPONSIVE APPARATUS Original Filed June 14, 1943 3 Sheets-Sheet`l H2.; mi Hm CIZ (R33 x April 6, 1948 Y yLL HAMMoND ErAL 2,439,295

SPEED RESPONSIVE APPARATUS Originai Filed June 14,V 1943 3 Sheets-Sheet2 i SMNNI NNN awww. w

NLT

XW?. ,swt

, .S 8N www $2 wQ QN 4 QQSG m3 t A 5w a3* MQ sbt Y xh l NQ Y Y u" Q Nm QQW\ RY \m\{ fmuh I' In 4 a dft/ok?? Za/2er *5f/ l' if v April te, 194s.

.1.. HAMMoND Er AL SPEED RESPONSIVE APPARATUS 5 Sheets-Sheet 3 OriginalFiled June 14, 1943 and ane/t ,my

.i I Jr l l T l are/asA/amm Jo/2f? M nl' l I I l I l l I I I l l I l I lI I I l l l I l l I l I I 'Il "lll llllllllllllllllllllllll l||l|hd||l|l|HfHlll| ...all llll {Ill} A..t|||....|l||I||||||||||...||||L n A lll .MJ lllllllllllllllllllllllll...l l l |||l|v|.|\ l a l e l l l n n l -www l I l..lllllllllllllllllllllllllll IIL l|.| lll l l Il Il Il |||l. l II-II". II I l l l l l I I l l I I l l l I I l| |I l l Iii Il. l Ill-n I I l l lI l i I l I i I I I l I I I I I I.- nlIIIu'l l H` 11:1.. .MHHHHHHHHHHH.l

Patented-Apr. 6, 1,4948

SPEED nEsPoNsrva APPARATUS Laurens Hammond, Chicago, and John VM.l-lanert, Park Ridge,` Ill., assignors to Hammond VInstrument Company,Chicago, Ill., a corporation ot Delaware original application June 14,194s, serial No.

490,746. Divided and this application March '10, 1945, Serial No,582.135

Our invention relates generally to speed respon- 4 claims.'v (ci.117-311) sive apparatus and more particularly to means for indicatingthe angular ground speed for aircraft.

plate voltageindicated asfa terminal +90 v., through load resistors R28.The control grid 30 of tube I6 has a. series lgrid resistor R32 inaddition to the grid resistor RI4. The screens 34 of the tubes I6 to I9are connected to a suitable source of screen voltage indicated as aterminal +45 v.

The measurement of ground speed o f an air-y The blocking condensers Cl2couple the output lplane is in general a rather difiiult problem andplate circuits of eachl of these tubes to the sucusually requires agreat deal of expensive preciceeding tube of the cascaded series. Thesupsion apparatus if reasonable accuracy is to be lpressor grids 36 oftubes I6 to I9 are connected attained. It has usually been determinedbycalto the cathodes 22. culation involving a determination of air speed,104 The tune 20, operating substantially as a triode,

. altitude, drift and similar factors. In some informs a low impedancedriver and has a cathode stances it is not necessary to determine theactual 38 connected to ground through a self-bias resisground speed, butit is sufficient to determine the tor R46, while its screen grid'42 isconnected to angular speed of the plane relative to a point on the plate56, the latter being connected to a terthe terrain beneath it. l5 minal44, the potential of which is controlled by The apparatus of ourinvention may be utilized potentiometer 46 connected between ground andto determine such angular speed directly, so that a source of screen andplate voltage, indicated as if the altitude of the aircraft is known,the ground a terminal +150 v. The adjustment oi the pospeed may bequickly calculated. tentiometer 46 is made s`o as to have the voltage Itis thus an object of our invention to provide upon the screen 42 andplate 50 at +135 v. under an improved apparatus for determining theanguthe given conditions. Upon changes in line voltlar speed at which anairplane moves relative to age the slider of the potentiometer 46 isadjusted the terrain. L to bring the screen voltage back to its proper Afurther object is to providean improved indi'- value, and forconvenience in making this adjustcator for the ground speed of aircraft,or for indiment a voltmeter 48 is preferably provided, its catlng theangular-speed of any other objects. terminals being connected betweenground and Other-objects will appear from the following the slider ofthe potentiometerv 46. Any other description, reference being had to theaccornsuitable means for controlling the voltage of the panying drawingsin which: terminal 44, to compensate for changes'in line Figure l is a.circuit diagram of the frequency voltage could, of course, besubstituted for the measuring portion of the apparatus; manuallyadjusted potentiometer 46. The` sup- Figure 2 is a circuit diagram of amodified form pressor grid 54 of this tube 20 is internally conof thespeed responsive apparatus; and nected to the cathode 38 thereof.

Figure 3 is a diagram of the optical portion of The plate 56 hasconnected thereto a plurality the speed responsive apparatus shown inFig. 2. of condensers C66, C61, C62 and C63, the other Thisapplicationis a division of our copending terminals of these condensersbeing connected to application Serial No. 490,746, filed June 14, 1943,switch points of multi-contact switch 64. The which has matured intoPatent No. 2,408,930, isswitch arm 64 is connected to the plate 66 of asued October 8, 1946. diode 68 and to the cathode I6 of a diode 12. The

Referring to Fig. 1 the frequency measuring 40 plate 'I4 of the diode I2is connected to a suitable portion of the apparatus 'may comprise adistortconstant potential source indicated as -3 v. The ing andamplifying system having input terminals cathode 'I6 of the diode 68 isconnected through I0, II which are resistance coupled through a a directcurrent milliammeter 'I8 to ground.

. blocking condenserCI2, across a grid resistor RI4, The plate is alsoconnected by a conductor to the input of yan electron discharge deviceI6 45 8Il,b1ocking condenser C82 and a decoupling resisforming' the rstof a cascaded series of distorting tor R84 to amulti-contact switch arm86 and and amplifying tubes. In addition tothe tube I6 also to thecontrol grid 88 of a pentode 90.

the successive stages of this distorting and am-v Y The pentode 90,which may be of the 6K6GT plifying system include tubes I1, I8 and A.I 9and a type, comprises a cathode 92 connected to a juncpower driver tube20. The tubes I6 to I9 may 50 tion 94 between voltage dividing resistorsR96 and j be pentodes ofthe 6SJ'7 type while the tube 20 R91. Thesevoltage dividing resistors are prefy may be of the 6K6GT type. Thecathodes 22 of erably wire wound resistors oi low value. The the tubesI6 to I9 are Arespectively connected to, resistors R96 and R91 areconnected in series ground through self-bias resistors R24 while theirbetween ground and the terminal 44, which, as plates 26 are connected toa .suitable source of 55 previously noted, is adjusted to be maintainedat l+ volts. The screen grid 98 of the pentode 96 is likewise connectedtothe terminal 44 while the spect to suppressor grid is internallyconnected to the cathode 92. The plate |02 of the pentode 90 isconnected to the terminal 44 through a loadA resistor RIM and is alsoconnectedby a conductor |06 to terminals of condensersl CI08, CI09 andC||0, the other terminalsv of these condensers being respectivelyconnected to switch points engageable by the switch arm 64.

A plurality of tuned meshes H2, ll3 and ||4 are adapted to be connectedselectively across the input of the pentode 90 by operation of theswitch 86. Each of these meshes ||2 to H4 comprises a condenser andinductance connected in parallel to provide impedances varying withfrequency. For example, the mesh ||2 may be tuned to resonate at 40 C.P. S., the mesh I I3 at 75 C. P. S.. and the mesh IM at 1,250 C. P. S.It will appear hereinafter that these frequencyresponsive meshes areutilized when measuring frequencies close to 32, 60 and 1000 C. P. S.respectively. a

In utilizing the instrument the frequency to be measured-is impressedacross the input terminals I0, Il. The input signal. with the degree ofamplification and distortion as shown,

should exceed 5 millivolts. Due to the non-linear properties of thesystem, any voltage above this value. Presumably the frequency to bemeasured is a sine wave, or a wave closely approaching a sine wave andsuch input wave is indicated at |20. The input wave may be of saw-tooth,rectangular, triangular or of any other generally symmetrical shape.

The sharp cutoff tubes I6 to I9 of the'6SJ7 type. supplied with thepotentials indicated, operate in a class A manner with a gain of 40 onlywhen their grids are supplied with very small signals. Thus it isapparent that for very minute signals upon the control grid of the tubeI0, the cascaded series of tubes I6 to 20 constitute a very high gainthe input signal is increased this cascaded system operates nonlinearlyand limits the amplitude of the signal as delivered by the plate of thetube 20 to a constant value.

This limiting action is made clear by reference vtoV the representativeoutput waves |20'to |25 of the ^d'iiererit tubes, shown directly abovethe tubes. The wave |2| is substantially a sine wave since it is assumedthat the input signal is the lsignal may be oi?` amplification system.When 4` ohm plate resistor R28. the distorted input wave becomessymmetrical about the horizontal axis. The second stage of amplificationcomprising the tube II, operates in a similar manner, but since theinput signal to this tube is presumably of greater amplitude, this tubewill introduce appreciable distortion in the signal, and its outputwaves will be generally of the shape indicated bythe wave |22. Similarlytubes I0 and I9 forming parts of the thirdand fourth stages ofamplification, will further amplify and distort the signal waves to morepronounced rectangular shapes, such as illustrated by the waves |23 and|24. i

The final stage of amplification,l which includes the driver tube 20,will further distort the wavey to a substantially perfect rectangularshape as indicated by the wave |25.

In the foregoing description of the operation of the ampliiler it wasassumed that a sine wave signal was being amplified.. Because of thefact that the tube 20 has its output limited in amplitude, and becausethe gain of the amplifier is very high, a signal of very small amplitudesup plied to the input of the tube I6 will produce a wave, such as thewave |25, of maximum amplitude in the output of tube 20. Furthermore,

anyinput wave of higher. amplitude will likewise produce a wavesubstantially identical with the wave |25. Thusthe output of theamplifier is substantially the same, particularly as to the lower orderharmonics the'root-mean-square value, irrespective of the amplitude ofthe input signal. As is well known to those skilled in the art, theharmonic series of a rectangular wave shape'such as the wave |25 may berepresented by a Fourier series of a fundamental and odd numberedharmonics. The third harmonic has an amplitude one-third of theamplitude of the fundamental; the ilfth harmonic has an amplitudeone-fifth of the fundamental; the seventh harmonic has an amplitudeone-seventh of the fundamental; etc. For

relatively small andthe tube l0 is not driven far beyond the point atwhich it operates non-linearly. However, in the event that anexceedingly high input signal is present, series resistor R32, which isof high value, such as 2 megohms. serves to limit the extent to whichthe signal may swing the control grid 30 in a positive sense. As soon asthe grid tends to go positive with rethe cathode, a voltage divisionoccurs between the resistor R32 and the cathode-togrid input impedanceof the tube I6, which will be much less than 2 megohms. Thus, furtherpositive increases in the signal voltage have 'neg-v driving the controlgrid 30 in a ligible effect in more positive direction.

The resistor R24 maybe of 4000 ohms. Un-

der these conditions the average plate 'current` will remain constantregardless of the input signai amplitude. When the signal drives thegrid positive,-as explained above, to -produce a fiattening of thepositive portions of the signal wave, the relatively sharp cutoff pointof the tube I6 will cause a corresponding flattening of the negativeportion 'of lecting a 4000 ohm self-bias resistor and 100,000

the signal wave. By seably affect the root-'mean-square value. ,therewill be no appreciable changes in their ama true rectangular wave theharmonic series is thus of infinite extent,` which, of course, is notachieved in practice. However, the values of the fundamental and lowerorder odd harmonics and of the root-mean-square value are substantiallyindependent of thev exact degree of steepness of the sides of thegenerally rectangular wave.

As will be apparent by comparison of the output waves of the successivestages of amplification, the wave becomes of more nearly truerectangular shape as the signal is transmitted through the successivestages. These changes in wave shape, after the wave has attained agenerally rectangular shape, e. g., wave |23, result mainly in anincrease inthe amplitudes of the higher order harmonics of'the Fourierseries but do not appreciably affect the amplitude of the lower orderharmonics nor do they appreci- Thus plitude in the output of the tube 20provided the input signal is above the predetermined minimum value of.005 volt. It is thus apparent that the signal as delivered by the lastof these tubes -is strikingly independent of the amplitude and waveshape of the input signal, and large changes V thereof have noappreciable affect upon the amplitude and root-mean-square value of theoutput signal. i

Assuming that the approximate frequency of the input signal is notknown, the operator will move to the switch arm 64 to the position suchof the output signal and i,

5 that the condenser C63 is in the circuit. v The output wave |26 willthus be transmitted to the diodes 68 and 12 which operate as a half-waverectifier to transmit the energy of the positive portion of the wave tothe milliammeter 18. The ammeter is preferably of good sensitivty suchas provided by a zero to one `mllliampere range. The value of thecondenser C63 is such that when the switch 64 connects this condenser inseries in the circuit and the input signal has a frequency of 10,000 C.P. S. the milliammeter 18 will indicate its full s cale'one milliamperereading. Assuming that the input frequency is 8 C. P. S., the deflectionof the needle of the ammeter 18. will be extremely small under the givenconditions. The operator will therefore shift the switch arm 64 to bringthe condenser C62 in the circuit, this condenser being of such valuethat an input of frequency of 1000 cycles will produce full scaledeflection of the the needle of the ammeter 18. At the assumed inputfrequency'of 8 C. P. S., the ammeter 18 will still fail to provide anappreciable deflection and the operator will therefore shift the switcharm 64 to bring the condenser C6| in the circuit. This condenser is ofsuch value that an input frequency of 100 C. P. S. will cause full scaledeflection of the meter 18. Under thse circumstances the 8 cycle inputfrequency will produce a deflection of the indicator of the ammeterwhich will be readable. The operator will note that the reading is belowthe 0.1 mark of the ammeter scale and will therefore make the finalshiftrof the switch armv l 64 to the contact which brings the condenserC50 in the circuit. The condenser C68 isof such value that an inputfrequencyof 10 C. P. S. will produce full scale deectlon of the meter18.

When the apparatus has thus been brought to final adustment the operatorwill read directly from the meterl the value of approximately 0.8 whichwill, of course, be interpreted as a frequency of 8 C. P. S. Ouf course,the meter 18 will in practice be frequency calibrated so as to bedirect-reading.

If the operator knows the approximate value of the frequency beingmeasured, it will not be k necessary for him to successively shift theswitch arm 64 as described but he may set it immediately to the rangewithin which the frequencylies. p

When the switch 64 is in its most sentitive position for a given inputfrequency (the setting with the condenser C60 in circuit in the aboveexample) the4 current drawn through this condenser, the diodes 68and'12, and meter 18 constitutes a load upon 'the output of the drivertube 20 and this will result in some distortion of the output wave |25.The diode' '12 effects the dischargel of the condenser C68 as well asproviding a low impedance shunt path -3 v. for Jthe negative portion ofthe output wave. l

When the input frequency is known to be very closeto a certainfrequency, such as is the casein power linekfrequenoles, the appropriatecondenser C|08, C|08 or C| I8 is connected in the circuit by the switcharm 64 and correspondingly, the

i Assuming that the input frequency is known to be approximately 32 C.P. S., greatly increased sensitivity of the meter may be obtained bymoving switchA arm 86 to a position to connect mesh I2 in' the inputcircuit of the pentode 88 and at the same time moving switch arm 64 tobring the appropriate condenser C|88 in the meter circuit.

As described above, the mesh l I2 is designed to be resonant at 40A C.P. S. The condenser C|08 is of such value that when it is connected inthe circuit, and an input frequency of 35 C. P. S. applied, the meter 18reads its full scale value of one milliampere. For frequencies less than35 C. P. S. the meter reading will drop very rapidly due to the effectof the threshold bias which is developed in the voltage divider resistorR86. By increasing the amount of this threshold bias on the control gridof the tube 80, the sensitivity of the frequency meter increases veryrapidly. Inasmuch 'as the frequency meter in this condition if effectiveto measure the fundamental component of the distorted signal ywave |25,changes in input 'amplitude at the terminals I 0, are substantiallyineffective to change the amplitude of the fundamental of the complexwave |25. However, small changes in -signal frequency result incorresponding changes in amplitude of signal occurring across the mesh II2 which is connected to the control grid 88. These relatively smallamplitude changesA causes a very large change in the average platecurrent becauseof the presence of the threshold bias as provided by thevoltage drop occurring in the resistor R86. In using the frequencymeter', advantage is taken of the steeply rising portion of theimpedance characteristic of the mesh ||2 as the input frequencyapproaches resonance. For instance, if the frequency range is to lieabout 32 C. P. S., the resonant frequency of the mesh ||2 is set at apoint such as 40 C. P. S. under which conditions measurements may bemade of frequencies between28 and 35 C. P. S. Because of the thresholdbias on the rectifier tube 80, frequencies below a predetermined minimumsuch as 28 C. P. S. are ineffective toproduce appreciable plate currentpulsations., Thus, by using the correct amount of threshold bias, thefrequency range of the meter may be-compressed to any extent desired.For measuring commercial power frequencies, such as 60 C. P. S., theswitch arm 64 is set to connect with condenser C |08 and the switch arm86 simultaneously set to connect mesh H3 (tuned to approximately C. P.S.) in theinput circuit of tube 90. For measurement of frequencies inthe 1000 C. P. S. range. the switch arm 64 may be connected to condenserH8 and the switch arm 86 set to connect mesh ||4 (tuned to approximately1250 C. P. S.) in the input circuit of tube 80.

The circuit and mechanism shown in Fig. lmay be utilized as a part ofthe apparatus illustrated in Figs. 2 and 3. In these gures thefrequenecy responsive circuit forms part of an angular speed indicator,as forexample indicating the ground speed of aircraft.

In this use of the invention a pair of phototubes |30, I3| are mountedin`a suitable case |32 having a dividing partition |33 therein. A pairof light grating plates |34, |35 of glass or other' suitable transparentmaterial are mounted in spaced relation in the case |32, the vplates'being accurately positioned and clamped by any suitable means so as tolie in parallel planes.

The grating plates |34, |35 have alternate opaque and transparentportions extending in parallel lines transversely thereof. A simplemethod of making these gratings is to coat sheets of plate glass with anaqueous solution of colloidal graphite. After this coating is dried andset, portions thereof are removed therefrom as by scrapingit, using amilling or other precision machine for accurately spacing the lines andassuring their parallelism. For example the colloidal graphite lines maybe in the order of .020" in width and may extend the full width of theglass plates. In a particular embodiment of the invention the ruleportions of the plates |34, |35 were approximately 3.5" by 7" and thespacing between the plates was 2". In Fig. 8 the opaque lines ofcolloidal graphite upon the `platea |33 and |35 are indicated by theheavy dash lines |33.

The casing |32 is mounted on the airplane so as to have a clear viewdownwardly therefrom, and as a result, rays of light from the groundwill pass through the transparent portions of the plates |34,` |35.However, it will be noted that in ,the lower half of the grating |35 theopaque lines |36 'are staggered with respect to the opaque lines of thegrating |36 in a manner such that rays striking the grating |35perpendicularly and passing therethrough will :be stopped by the opaquelines of the grating |34. On the other hand, rays striking the upperportion of the grating |35 perpendicularly and passing through thetransparent portions thereof will also strike and pass through thetransparent lines of grating |34.

This is indicated in Fig. 3 by the fact that perpendicular rays |40 arestopped by the lower portion of the grating |3| and thus do not have anopportunity to affect the phototube |3|, while such rays striking theupper portion of the grating |35 pass through the transparent linesthereof and also pass through the transparent lines of grating |34 andmay thus fall upon the-sensitive surface of the phototube |30.

On the other hand rays of light |3| striking the lower portion of thegrating |35 at certain angles pass through the transparent portionsthereof .and also strike and pass through the transparent lines ofgrating |34 and thus may reach the sensitive surface of the phototube|3|. Rays |4| passing-through the upper portion of grating |35 arestopped by the opaque lines of the grating |34.

Thus, as the apparatus moves in the direction represented by the arrow|44, or in the opposite direction, a stationary source of light wouldalternately energize the phototubes and |3|. Since, as shown in Fig. 2,the cathode of phototube |30 and the plate of phototube |3| areconnccted through capacitor CH3 and the grid resistor to the controlgrid |50 of the preamplifier pontode |52, an increase in theillumination of phototube |30 will cause the potential on the grid |50to decrease, while an increase in the illumination of the phototube |3|will cause an increase in the potential of the control grid |50. Duetothe positions of the gratings. the probabilities are that a light source(having relative angular movement with respect to the gratings) willalternately 4 apply increased illumination to thephototubes |30, |3|.Therefore the output signal due to scanning a single light source wouldtend to be a sine wave of approximately twice the amplitude of the wavewhich would be produced if,

but a single phototube and a single set of gratings were employed. f

Because of the large number of light slits, a single point source oflight being scanned will produce an extended series of generally sinewave impulses, thereby greatly improving the reliability with which thefrequency of the signal may be determined. With the gratings of thedimensions indicated above, a complete cycle of energization grating ofthe plates |35 traverse the grating of A phototube I3 of the phototubes|30, |3| would take place as the light source andthe gratings shiftrelatively through an angle in the order of 110. Consequently therelative angular speed of a light source and the phototubes may bedetermined by measuring the frequency generated by the phototubes, andthe relative linear speed may be computed if the distance from thesource is known.

In utilizing the apparatus as a ground speed indicator for aircraft thealtitude of the plane may be quite accurately determined by conventionalinstruments and thus the frequency at which the phototubes |30, |3| arealternately energized will constitute a measure of the ground speed ofthe plane. There will, in nearly all cases, be sum- `cientirregularities or discontinuities in the level of illumination ofdifferent portions of the ground to differentially energize thephototubes |30 and |3|. Of course, if all portions of the terrainreflected light uniformly, no signal would be generated in thephototubes, but experience has shown that the, intensity of lightradiation from adjacent portions of the terrain vary considerably eventhough to the eye they may appear to be relatively uniform. Thus anyslight irregularities such as a bush, a tree, a fence or a Whitecap mayproduce suilcient discontinuity in the intensity of radiation from thefield scanned by the phototubes, vto produce a significant signaltherein. The illumination of the phototubes |30, |3| as they scan asource of illumination, produces electrical signal waves which aregenerally triangularr shape, corresponding to the linear increase anddecrease in illumination as rays from the light source which have passedthrough the plate |34. The signals produced by the two phototubes willbe 180 degrees out of phase because of the fact that the gratings|34'and |35 in front of the phototube |3| are staggered, while thegratings in front of the phototube |30 are ln alignment.

As best shown in Fig. 2 the phototubes |30, |3| are connected in series,the cathode of the phototube |30 being connected to a suitablesource ofnegative potential, as a terminal -45 v., while the anode thereof isconnected to the cathode of The anode of the latter phototube isconnected to a suitable positive potential source, indicated as aterminal +45 v. The anode of phototube |30 and cathode of phototube |3|are connected through a blocking condenser CMG and radio frequencyfilter resistor R|43 to the grid |50 of an amplifier tube |52 which maybe of the 6.17 type. A grid resistor R|54 of relatively high value isconnected in series with the resistor R|48 in the input circuit of thetube |52.

The cathode |56 of tube |50 is connected to ground through a self-biasresistor R|58. The screen grid |59 and suppressor grid |60 are connectedto the plate |6| so as to cause the tube to operate as a triode in alinear manner. A plate load resistor R|62 is connected between the plate|6| and a suitable plate voltage source indicated as a terminal v.

Because tube |52 is utilized in the circuit as l a triode it offers theadvantage of low plate impedance, and because of its high impedanceinput circuit this tube is preferably located Within or immediatelyadjacent the casing |32 and is resilp iently -supported to avoid theintroduction 0i microphonic disturbances.

The output of the tube |52 is transmitted through a Vshielded conductor|64 to a blocking vvFig. 1.

condenser C166. 'Il'he grid |68 of an amplifying and distorting pentodeis connected to the condenser CI66 through a series grid resistor R32and is connected to ground through the grid resistor RI4. The pentode|10 corresponds in function and in its associated circuit elements withthe tube I1 of Fig. l and is coupled to a tube |12 which corresponds tothe tube I8 of The coupling is through a sensitivity controlpotentiometer RI14.

Plate |16 of the tube |12 is connected through voltage dividing loadresistors R|18 and RI.80 to a +90 v. terminal. The junction of theresistors R|18 and RI80 is connected through a blocking vcondenser C|62and a series resistor R|84 to the grid |86 of a pentode |88 which may beof the 6SJ7 sharp cutoff type. The pentode |88 is provided with a gridresistor RI4 and a self-bias resistor R24. The other electrodes of thepentodes |88 are connected to suitable fixed potential sources, and theoutput signal, which is a`wave rectangular shape corresponding to thewave |25 of Fig. 1, is transmitted through a blocking condenser C|90 tothe input circuit of a pentode |92, through a current limiting gridresistor RI94. A grid resistor R|96 in series with the resistor RI94 isconnected to asuita-` ble bia-sing potential source indicated as aterminal 22.5 v.. which constitutes a large negative grid bias for thistype of tube, so that this tube operates in the manner of the grid con@trolled rectifier having a high input threshold.A The plate |98 of thepentode |92 is connected through a plate load resistor R200 to asuitable plate voltage source, indicated as +45 v., to which the screen20| of this tube is also connected. A by-pass condenser C202 isconnected in shunt with the load resistor R200. The plate |98 is alsodirectly connected with the grid 204 of a pentode 206. The cathode 208of the tube 206, as well as its suppressor grid 209, is connected `to a+45 v. terminal. The plate 2|0 of the pentode 206, which is a power tubeand may be of the 6K6GT type, is connected through the y winding of arelay 2I2 to a plate `voltage source indicated as a terminal +90 v. Thescreen grid' 2I3 of this tube 206 is ter terminal.

It will be seen that when the pentode |92 is not conducting current thevoltage on the grid 204 of the pentode 206 will be substantially +45 v.and hence the latter tube will be conducting and maintain the relay 2I2energized. When, however, the pentode |92 is conducting the signalimpulses the voltage drop across the load resistor R200, due to theresultant increase in plate current flow through the pentode |92, willbe suilicient to cause the grid potential bias on the tube 206 to dropto substantially a cutoi value also connected to the latand thus causethe deenergizatlon of the relay The relay 2I2 upon energization closes aswitch 2|4 in a circuit which Iincludes a singlerole double throwrelayswitch 2| 6 and a sultable operating current source indicated as ateramplitude signal which will cause the formation of a true rectangularwave shape in the output of the pentode |88. Unless such wave is ofsubstantially true rectangular shape. the signal for actuating the relay2I2 will not be thoroughly reliable and it is therefore desirable toprevent the effective operation of this relay under such circumstances.The voltage in the output signal of the pentode |12 is transmittedthrough a blocking condenser C224 to the grid 226 of pentode 228. Abiasing threshold determining resistor R230 connects the srid 226 to asuitable biasing potential source indicated as a terminal 22.5 v. Theoutput of the pentode 228 is coupled to the input of a power pentode 230which operates, in a manner similar to the tube 206, to conduct wheneverthe pentode 228 is not conducting anl appreciable signal and to be cutoi when the pentode 228 is drawing substantial plate current.

The lower contact ofthe switch 2I6 is connecte ed through an indicatorlamp 232 to a +6 v. terminal, and a visual indication of the operationof relay 222 thus provided.v

The relay 2 I8 is adapted to control the operation of a reversiblepermanent magnet heldv motor 234, which is provided with a suitablespeed governor. and is shunted by an anti-spark rey sistor 236. Therelay 2I8 has single-pole doublethrow switches 238 and 239 which when inthe vupper position shown connect a conductor 240 minal of which isgrounded.

minal +12 v ,land also the winding of a relay y 2IB, which is shunted byR220.

The switch 2I6 is operated by a cut-out relay 222. `The relay 222Ioperates, as will hereinafter appear,to prevent eiective utilization ofthe actuation of the relay 2 I2 whenever the amplitude of the signaloutput of the pentode |12 falls below a predetermined minimum value.This minimum value is determined by the minimum an anti-spark resistoreiectively blocking this tube.

relay 2| 2 remains deenergized and the relay 2| 8 4 Similarly when therelay 2I8 is energized, an energizing circuit for the motor 234 isestablished through a circuit including the conductor 240 (at +12volts), the switch 239, motor 234, switch 236, conductor 243 and limitswitch 244 to ground. The limit switches 242 and 244 are operativelyconnected to the armature yof motor 234 and open its above describedenergizing circuits when the motor is driven in one direction ortheother bcyond predetermined limits.

'Ihe motor 234 is connected through a suitable speed reduction gearingtoV the adjustable element of a variable condenser C246. the latter'be-4ing connected between ground andthe current limiting resistor R|94associated with the input grid of tube |92. The condenser C246 inconjunction with the current limiting grid res-'Stor RI94, forms afrequency responuve for controlling the amplitude of the signalimpressed upon the circuit of the pentode |92.

To explain the function of the motor operated variable condenser C246,it will be assumed that the input frequency initially generated in thephototubes I 30, I 3| decreases in value. Such decreased frequencysignal, after passing through the amplifying and distorting systemcomprising the tubes |52, |10, |12 and |88, willcause an increase in theamplitude of the signal across` the input of the tube I 92. 'Thisincrease in the input signal amplitude to thetube |92 will usuallyexceed the threshold determined b y the 22.5 v. bias, and the pentode|92 will thus be rendered conducting, and as a result, increasenegatively the bias upon the grid of the power tube 206, As a result thelikewise deenergized. Deenergization of the relay ZIB, through thepositioning of the switches 238 dered conducting and energizes relay 2|and 239, reverses the polarity of the motor 234 and caues it to drivethe condenser C246 in a direction to cause its capacitance to increase.As a result of the increased capacitance of the condenser C246. there isa corresponding decrease in the amplitude of the signal at the input oftube |92. The resulting decrease in current flow through this tube |92causes an increase in the potential of the grid 204 and the tube 206 isrenlay 2|2 then energizes relay 2|8, and the latter, by moving switches238, 239 to their dotted line positions, causes the motor 234 to rotatein a reverse direction, i. e., in a direction to decrease thecapacitance of condenser C249. Whenever the amplifier is receiving asignificant signal the motor 234 is operating either in one direction orthe other. This is of advantage in that `the condenser C246 isat alltimes increasing or decreasing its capacitance, such increase anddecrease being effective to tune the input circuit for the pentode |92to a frequency alternately slightly above and slightly below that of theinput signal.

l The mean of these two frequencies will be that of the input signalwith a high degree of accuracy. Since the motor is operating at alltimes, there is no lag in the response of the instrument. This isbecause the circuit elements, particularly the condenser' C246 andseries grid resistor Ri 94, may be so designed that the two frequencies,between which the frequency response is varied, may be relatively closeto one another with the result that the motor 234 will reverse itsdirection of rotation at very short intervals, in the order of a secondor two. The drive from the motor to the movable part of the condenserC246 will ordinarily be such that the movable part of the condenser willVoscillate through such a small arc that the oscillations will hardly benoticeable. Thus it willmbe seen, the position of the movable part ofthe variable condenser C246 will constitute an indication of thefrequency generated by the phototubes |30, |3l.

'Any suitably calibrated device may by the motor 234 to provide amechanical movement responsive to the angular speed of the airplanerelative to the terrain below it, or to provide a direct readingindication or record of the frequency generated by the phototubes |30,|3|,

or of the factor which combined with the altitude will show the groundspeed.

Whenever the signal generated by the phototubes is of such low amplitudeas to lack significance, the tube 228 becomes biased substantially to orbeyond cutoff and the pentode 230 is thereby rendered conducting andenergizes relay 222. Energization of relay 222 completes the circuit tothe warning signal lamp 232, and opens the circuit by which power issupplied to the lmotor 234. The motor 234 can therefore no longer becontrolled by the relay 2|3 and the contact 2|4, and any frequencyindicating means operated by the motor 234 will remain stationary untilthe phototubes |30, |3| again supply a signal of significant amplitude.'f

When the airplane on which the apparatus is mounted is flying over` anordinary terrain, the apparatus will provide a continuous indication ofthe angular speed at which the ground is passing directly beneath theplane. To obtain increased accuracy. the casing |32 containing thephototubes |30, |3| may be mounted upon a gyro stabilized support, sothat the phototubes always receive light from an area directly beneaththe airplane and so that minor irregularities in the flight path orattitude of the airplane will not appreciably affect the operation ofthe apparatus;

While we have disclosed particular embodiments of the invention, it willbe apparent to` those skilled in the art that numerous 'variations andmodifications of the invention may be made without departing from theunderlying principles thereof. We therefore desire, by the accom-`panying claims, to include within the scope 0f our invention all such.variations and modifications by Which substantially the results of ourinvention may be obtained through the use of substantially the same orequivalent means.

We claim:

1. In an angular ground speed responsive apparatus for aircraft, thecombination of a pair of phototubes, a pair of spaced gratings in frontof each of said phototubes, said gratings being formed to cause parallelrays of light shifting angularly with respect to said phototubes toenergize said phototubes alternately, a non-linear amplifier coupled tosaid phototubes capable of amplifying and distorting the signal producedby said phototubes into a signal of substantially rectangular wave shapeand of constant amplitude, and a frequency responsive means coupled tosaid amplifier.

' 2. In angular ground s peed responsive apparatus for aircraft, thecombination of a pair of phototubes, a pair of spaced gratings in frontof each of said phototubes, said gratings being formed to cause parallelrays of light shifting angularly with respect to said phototubes to enbedriven ergize said phototubes alternately, and a frequency responsivedevice coupled to said phototubes and operable to provide an indicationoi.'

the frequency of the signal produced thereby.

' 3. In an angular ground speed responsive apparatus for aircraft, thecombination of a pair of phototubes, a pair of spaced gratings in frontof each of said phototubes, said gratings being. formed to causeparallel rays of light shifting angularly with respect to saidphototubes to energize said phototubes alternately, a non-linearamplifier coupled to said phototubes capable of amplifying anddistorting the signal produced by said phototubes into a signal ofsubstantially rectangular wave shape and of constant amplitude, andfrequency responsive means coupled to the output of the amplifier l 4.In angular ground speed responsive apparatus for aircraft, thecombination of a pair of phototubes, a pair of spaced gratings in frontof each of said phototubes, said gratings being formed to cause parallelrays of light shifting angularly with respect to said phototubes toenergize said phototubes alternately, and means coupled to saidphototubes to amplify the signals produced thereby to a predeterminedconstant amplitude, and frequency responsive means coupled to theamplifying means.

LAURENS HAMMOND. JOHN M. HANERT.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Hammond et al. Oct. 8,

